Insulin affects the ability of Gi to be ADP-ribosylated but does not elicit its phosphorylation in intact hepatocytes

Receptor tyrosine kinase-G-protein-coupled receptor signalling platforms: out of the shadow?

Receptor tyrosine kinases (RTKs) and G-protein-coupled receptors (GPCRs) can form platforms in which protein signalling components specific for each receptor are shared (owing to close proximity) to produce an integrated response upon engagement of ligands. RTK-GPCR signalling platforms respond to growth factors and GPCR agonists to increase gain over and above that which is normally produced by separate receptors. They can also function to change the spatial context of signalling in response to growth factor activation. The function of RTK-GPCR signalling platforms can be modulated with conformational-specific inhibitors that stabilise defined GPCR states to abrogate both GPCR agonist- and growth factor-stimulated cell responses. In this paper, we provide an opinion of the biology and unusual pharmacology of RTK-GPCR signalling platforms and make comparisons with a more traditional model of crosstalk between RTKs and GPCRs.

The role of G-protein coupled receptors and associated proteins in receptor tyrosine kinase signal transduction

It is well established that stimulation of G-protein coupled receptors (GPCRs) can activate signalling from receptor tyrosine kinases by a process termed transactivation. Indeed, in recent years, it has become apparent that transactivation is a general phenomenon that has been demonstrated for many unrelated GPCRs and receptor tyrosine kinases. In this case the GPCR/G-protein participation is up-stream of the receptor tyrosine kinase. Substantial research has addressed these findings but meanwhile another mechanism of cross talk has been slowly emerging. For over a decade, a growing body of evidence has demonstrated that numerous growth factors use G-proteins and attendant signalling molecules such as beta-arrestins that participate down-stream of the receptor tyrosine kinase to signal to effectors, such as p42/p44 MAPK. This review highlights this novel mechanism of cross talk between receptor tyrosine kinases and GPCRs, which is distinct from growth factor receptor transactivation by GPCRs.

Characterization of lymphocyte beta 2-adrenoceptor signalling in patients with left ventricular volume overload disease

1. Studies using animal experimental models have suggested that the beta2-adrenoceptor is uncoupled in association with alterations in the expression of G-protein-coupled receptor kinases (GRK) 2/3 in heart failure. However, the functional expression of the components of this pathway in human disease has not been fully elucidated yet. In the present study, we evaluated the possibility that the regulation of beta2-adrenoceptor signalling components in patients with left ventricular volume overload (VOL) depends on the severity of the overload. 2. We characterized the lymphocyte GRK 2-6, beta-arrestins 1 and 2, beta2-adrenoceptor expression at the mRNA and protein levels, as well as the activity of adenylyl cyclase, protein kinases (PK) A and PKC in patients with VOL using healthy blood donors as controls. 3. In the patient group, GRK2 mRNA was increased by 61% (P < 0.001), GRK3 was increased by 54% (P < 0.005), GRK5 was increased fivefold (P < 0.001) and the beta-arrestin 2 mRNA was increased by 40% (P < 0.05). These increases were paralleled with a sixfold increase in GRK2, a twofold increase in GRK3 and a 1.3-fold increase in GRK5 protein levels. These changes were associated with a significant decrease in beta2-adrenoceptor mRNA, the basal, catalytic and receptor-mediated activity of adenylyl cyclase and sensitization of the forskolin-stimulated activity towards augmented inhibition by guanylimidodiphosphate. In general, the increase in GRK2 and 5 mRNA exhibited a positive correlation with the gravity of the haemodynamic load, as determined by changes in left ventricular fractional shortening. 4. The results suggest that VOL induces an increase in the expression of lymphocyte beta2-adrenoceptor-specific GRK and beta-arrestin 2 in association with an attenuation in beta2-adrenoceptor levels. It can be speculated that the cardiac circulatory system adapts itself to altered haemodynamic functional demands partly by altering beta2-adrenoceptor signalling.

Association of heterotrimeric G(i) with the insulin-like growth factor-I receptor. Release of G(betagamma) subunits upon receptor activation

The insulin-like growth factor-I receptor (IGF-IR) is a key regulator of cell proliferation and survival. Activation of the IGF-IR induces tyrosine autophosphorylation and the binding of a series of adaptor molecules, thereby leading to the activation of MAPK. It has been demonstrated that pertussis toxin, which inactivates the G(i) class of GTP-binding proteins, inhibits IGF-I-mediated activation of MAPK, and a specific role for G(betagamma) subunits in IGF-I signaling was shown. In the present study, we have investigated the role of heterotrimeric G(i) in IGF-IR signaling in neuronal cells. Pertussis toxin inhibited IGF-I-induced activation of MAPK in rat cerebellar granule neurons and NG-108 neuronal cells. G(alphai) and G(beta) subunits were associated with IGF-IR immunoprecipitates. Similarly, in IGF-IR-null mouse embryo fibroblasts transfected with the human IGF-IR, G(i) was complexed with the IGF-IR. G(alphas) was not associated with the IGF-IR in any cell type. IGF-I induced the release of the G(beta) subunits from the IGF-IR but had no effect on the association of G(alphai). These results demonstrate an association of heterotrimeric G(i) with the IGF-IR and identify a discrete pool of G(betagamma) subunits available for downstream signaling following stimulation with IGF-I.

A selective inverse agonist for central cannabinoid receptor inhibits mitogen-activated protein kinase activation stimulated by insulin or insulin-like growth factor 1. Evidence for a new model of receptor/ligand interactions

In the present study, we showed that Chinese hamster ovary (CHO) cells transfected with human central cannabinoid receptor (CB1) exhibit high constitutive activity at both levels of mitogen-activated protein kinase (MAPK) and adenylyl cyclase. These activities could be blocked by the CB1-selective ligand, SR 141716A, that functions as an inverse agonist. Moreover, binding studies showed that guanine nucleotides decreased the binding of the agonist CP-55,940, an effect usually observed with agonists, whereas it enhanced the binding of SR 141716A, a property of inverse agonists. Unexpectedly, we found that CB1-mediated effects of SR 141716A included inhibition of MAPK activation by pertussis toxin-sensitive receptor-tyrosine kinase such as insulin or insulin-like growth factor 1 receptors but not by pertussis toxin-insensitive receptor-tyrosine kinase such as the fibroblast growth factor receptor. We also observed similar results when cells were stimulated with Mas-7, a mastoparan analog, that directly activates the Gi protein. Furthermore, SR 141716A inhibited guanosine 5'-0-(thiotriphosphate) uptake induced by CP-55,940 or Mas-7 in CHO-CB1 cell membranes. This indicates that, in addition to the inhibition of autoactivated CB1, SR 141716A can deliver a biological signal that blocks the Gi protein and consequently abrogates most of the Gi-mediated responses. By contrast, SR 141716A had no effect on MAPK activation by insulin or IGF1 in CHO cells lacking CB1 receptors, ruling out the possibility of a direct interaction of SR 141716A with the Gi protein. This supports the notion that the Gi protein may act as a negative intracellular signaling cross-talk molecule. From these original results, which considerably enlarge the biological properties of the inverse agonist, we propose a novel model for receptor/ligand interactions.

Pertussis toxin sensitive endothelin-1 coupling to inositol phosphate formation via a GTP-binding protein: comparison in SHR and WKY cultured aortic smooth muscle cells

The effects of pertussis toxin on endothelin-1 and noradrenaline coupling to inositol phosphate (IP) formation was investigated in cultured aortic smooth muscle cells from 14 week SHR and WKY rats. Endothelin-1 (10(-6) M) stimulated IP formation was decreased in cells from SHR compared to WKY (WKY 1117 +/- 157, SHR 668 +/- 85% of basal). Pre-incubation with pertussis toxin produced a significant and similar reduction in endothelin stimulated IP production in both SHR (54% reduction) and WKY (55%). However, the observed reduction in endothelin-1 stimulated IP accumulation was still apparent in SHR when compared to WKY. Pertussis toxin preincubation followed by removal of extracellular calcium reduced further the endothelin responses by similar amounts in SHR and WKY cells, but SHR stimulated IP formation remained significantly decreased compared to WKY. The extent of pertussis toxin ADP-ribosylation of Gi alpha was similar in both SHR and WKY cells. Endothelin-1 produced a reduction in the extent of ADP-ribosylation of Gi alpha and this was of similar magnitude in both SHR and WKY cell membranes. In contrast, noradrenaline stimulated IP formation was unaffected by pertussis toxin pre-incubation. It was concluded that SHR cells do not appear to have an alteration in endothelin-1 activated, pertussis toxin sensitive G-protein coupling to IP formation or in the dependence of inositol phosphate formation on extracellular calcium.

Guanine nucleotide binding regulatory proteins in liver from obese humans with and without type II diabetes: evidence for altered "cross-talk" between the insulin receptor and Gi-proteins

A novel pathway for physiological "cross-talk" between the insulin receptor and the regulatory Gi-protein has been demonstrated. We tested the hypothesis that a coupling defect between Gi and the insulin receptor is present in the liver of obese patients with and without type II diabetes. Insulin 1 x 10(-9) M (approximately ED50) and 1 x 10(-7) M (Max) inhibited pertussis toxin-catalyzed ADP ribosylation of Gi in human liver plasma membranes from lean and obese nondiabetic patients. However, 1 x 10(-7) M insulin was without effect in membranes from patients with type II diabetes. This coupling defect was not intrinsic to Gi, since Mg2+ and GTP gamma S inhibited pertussis toxin-catalyzed ADP ribosylation in both diabetic and nondiabetic patients. Binding of insulin of the alpha-subunit and activation of the tyrosine kinase intrinsic to the beta-subunit of the insulin receptor are not responsible for the coupling defect. 125I insulin binding is the same in obese patients with or without diabetes. Tyrosine kinase of the insulin receptor is decreased in diabetes. However, a monoclonal antibody to the insulin receptor (MA-20) at equimolar concentrations with insulin equally inhibits pertussis toxin-catalyzed ADP ribosylation of Gi without activating tyrosine kinase or insulin receptor autophosphorylation. Immunodetection of G-proteins suggested that Gi3 alpha was normal in diabetes and Gi1-2 alpha was decreased by 40% in the diabetic group as compared to the obese nondiabetic group but was normal when compared to the lean non diabetic group. We conclude that the novel pathway of insulin signaling involving the regulatory Gi proteins via biochemical mechanisms not directly involving the tyrosine kinase of the insulin receptor is altered in obese type II diabetes and offers a new target for the search of the mechanism(s) of insulin resistance.

Insulin-mediated sensitization of adenylyl cyclase activation

1. Insulin may be an important regulator of vascular function. We have previously studied lymphocyte beta-adrenoceptors as a model for the human vascular beta-adrenoceptor. To examine the effects of insulin on human beta-adrenoceptor responsiveness, adenylyl cyclase activity, cyclic AMP-dependent protein kinase activity and beta-adrenoceptor radioligand binding assays were performed on permeabilized mononuclear leukocytes. 2. With acute exposure to insulin in vitro, followed by washing and permeabilization there was a dose-dependent increase in both lymphocyte NaF-stimulated activity and beta-adrenoceptor-stimulated adenylyl cyclase activity paralleling an increase in beta-adrenoceptor-stimulated protein kinase A activity. Manganese-, forskolin- and forskolin plus guanylimidodiphosphate-stimulated adenylyl cyclase activities were not altered by insulin pretreatment. Additionally, mononuclear leukocyte beta-adrenoceptor density, proportion of externalized receptors and receptor affinity for agonist were not altered. 3. The data indicate that acute exposure to insulin sensitizes G-protein-stimulated adenylyl cyclase activity. These findings suggest a potential role for insulin in the regulation of beta-adrenoceptor responsiveness in man.

GTP-binding protein-activator sequences in the insulin receptor

Some functions of the insulin receptor (insR) are assumed to be mediated by pertussis toxin-sensitive Gi/G(o) proteins. Here we have located G-protein-activator domains in the cytoplasmic region of the human insR. We searched the sequence of insR and found three candidate regions at residues 1039-1061, 1147-1168 and 1325-1345, referred to as ISRP1, ISRP2 and ISRP3, respectively. Among them, the Gi/G(o)-activating function was observed only in peptide ISRP3. ISRP1 specifically activated Gs, whereas ISRP2 had no effect on G proteins. ISRP2 and ISRP3 contained five of six autophosphorylated tyrosine residues in insR. After tyrosine phosphorylation, ISRP2 showed specific Gi-activating function, and ISRP3 potentiated its ability and became capable of activating G proteins generally. This is the first study that specifies G-protein-activator domains in insR and describes their modification by autophosphorylation.

Alterations in G-protein expression, Gi function and stimulatory receptor-mediated regulation of adipocyte adenylyl cyclase in a model of insulin-resistant diabetes with obesity

The stimulatory effect of Mn2+ (1.5-fold), forskolin (1.6-fold) and low (1 microM) concentrations of GTP (1.9-fold) on the adenylyl cyclase of adipocyte membranes from obese, diabetic CBA/Ca mice was markedly enhanced compared to that seen using membranes prepared from their lean littermates. In contrast, receptor-mediated stimulation, achieved with either isoprenaline or secretin was reduced and that by glucagon abolished in membranes from diabetic animals. The levels of expression of alpha-subunits of Gi-1, Gi-2 and Gi-3 were reduced to some 49, 76 and 54%, respectively, in membranes from diabetic animals compared with those from normal animals. Levels of G-protein beta-subunits and Gs alpha-subunits were similar. Receptor-mediated inhibition of adenylate activity elicited by either nicotinic acid or prostaglandin E1 (PGE1) was of a similar magnitude in membranes from normal and diabetic animals but the inhibitory action of N6-(L-2-phenylisopropyl)adenosine (PIA) was greater in membranes from diabetic animals by about 30%. Gi function was similarly evident in membranes from both lean and diabetic animals, as assessed using low concentrations of guanylyl 5'-imidodiphosphate to inhibit forskolin-stimulated adenylyl cyclase activity. However, assessing Gi function using GTP showed marked dissimilarities in that the elevated GTP concentrations expected to occur physiologically were incapable of reversing the stimulation achieved at low concentrations of GTP in membranes from diabetic but not normal animals. The adipocytes of CBA/Ca mice, as do other animal models of insulin resistance, show lesions in adenylyl cyclase regulation, Gi function and G-protein expression.

Receptors for insulin interact with Gi-proteins and for epidermal growth factor with Gi- and Gs-proteins in rat pancreatic acinar cells

In rat pancreatic acinar cells epidermal growth factor (EGF) and insulin increase both basal and cholecystokinin (CCK-OP) stimulated amylase release in vitro (1) as a long term function of this tissue. Here we show that preincubation of isolated plasma membranes with EGF or with insulin leads to increased incorporation of the GTP-photoaffinity analogue [alpha-32P]GTP-gamma-azidoanilide into 40/41 kDa proteins and to reduction of pertussis toxin- (PT) catalyzed [alpha-32P]ADP-ribosylation of three 40/41 kDa proteins which had been previously identified as Gi1, Gi2 and Gi3 (2). In the presence of GTP gamma S, EGF- and insulin-induced inhibition of PT-mediated [alpha-32P]ADP-ribosylation of 40/41 kDa proteins is eliminated. EGF enhances cholera toxin- (CT) mediated ADP-ribosylation of all three 40/41 kDa Gi-proteins as well as of five 45 and four 48/50 kDa proteins, which had been previously identified as Gs-proteins (2), whereas insulin has no effect. We conclude from our data that both EGF and insulin interact with the same Gi-proteins as CCK-OP does, whereas EGF additionally interacts with nine Gs-proteins. It is likely that one, two or all three 40/41 kDa Gi-proteins are involved in insulin- and EGF-induced potentiation of CCK-OP-stimulated enzyme secretion. In addition interaction of EGF with Gs-protein could play a role in the potentiation of CCK-OP-induced enzyme secretion from pancreatic acinar cells.

Altered G-protein expression and adenylate cyclase activity in platelets of non-insulin-dependent diabetic (NIDDM) male subjects

Adenylate cyclase activity and levels of guanine nucleotide regulatory proteins (G-proteins) were compared in platelets from normal and non-insulin-dependent diabetic (NIDDM) male subjects. Whilst no differences were noted in basal and NaF-stimulated adenylate cyclase activities the degree of stimulation achieved by both forskolin and prostaglandin, E1 was lower by some 34 and 52% respectively, in platelet membranes from diabetic subjects compared with those from normal control subjects. Altered alpha 1-adrenoceptor-mediated inhibition of prostaglandin E1-stimulated adenylate cyclase activity was evident; it being some 34% lower in platelet membranes from diabetic subjects compared to controls. Analysis of G-protein alpha-subunits, using specific anti-peptide antisera, showed that platelets from all subjects exhibited the Gi-2 and Gi-3, but not the Gi-1 forms of the inhibitory G-protein 'Gi' and all expressed the 42 kDa species of alpha-subunit of the stimulatory G-protein Gs. Whilst platelets of diabetic subjects had levels of Gs which were comparable to those found in control subjects their levels of Gi-2 and Gi-3 were some 49 and 75%, respectively, of those found in platelets from control subjects. It is suggested that changes in adenylate cyclase functioning and G-protein expression may contribute to altered platelet functioning in non-insulin-dependent diabetic subjects.